A Fresnel Zone Plate (FZP), such as FZP 100 shown in FIG. 1, has a set of alternating transmissive and opaque concentric rings that serve as the diffracting elements to focus light. FZPs have been designed to operate at wavelengths that range from the radio wave range to the x-ray range. The ultimate resolution of a FZP is determined by the width of the outermost zone. Fresnel zone plates are attractive alternatives to refractive optics due to savings in size and weight. The main drawbacks for the FZP are the reduced transmission compared to refractive optics, severe chromatic aberration and the presence of secondary maxima (intensity variations in the form of concentric rings) that blur the image at the focal plane.
Photon sieves (PS) are planar imaging elements whose design is based on Fresnel zone plates (FZP), but performance in certain parameters is improved by replacing the transparent ring-shaped zones with an appropriate arrangement of pinholes as illustrated in FIGS. 2 and 3. An ideal lens would have an infinite diameter. An “apodized” lens, such as that of FIG. 3, creates a smooth transition of holes to the outer diameter of the lens to more closely approximate the performance of an infinite lens. FIG. 2 depicts an unapodized PS with 2,722 pinholes that is calculated to have a transparency of 55.8%. FIG. 3 depicts an apodized PS having 818 pinholes that exhibits a transparency of 28.3%. Each photon sieve is designed for a lens diameter of 1.0 mm, Focal Length=10 mm, and λ=650 nm.
Photon sieves use a quasi-random distribution of pinholes in a plane that replaces the conventional zone plate. The pinholes are generally approximately centered within a transparent zone of the underlying Fresnel zone plate. Photon sieves have multiple advantages over both refractive and Fresnel optics. Photon sieves are as compact, lightweight and easy to manufacture as Fresnel zone plates, but they form superior images in terms of sharpness and contrast. Generally, both photon sieves and Fresnel Zone Plates are diffractive devices that have a relatively narrow field of view, lower transmission, greater chromatic aberration, lower contrast and sharpness, compared to refractive devices (e.g., glass or plastic lenses). But photon sieves and Fresnel zone plates are nearly planar in geometry and are generally lower in weight than comparable refractive optics.